CHEM01200604005 A. K. Pathak - Homi Bhabha National Institute

including polarized and diffuse functions has been adopted for all calculations. Newton
Raphson based algorithm has been applied to carry out geometry optimization for each of
these molecular clusters with various initial guess structures to find out the most stable
configuration without any symmetry restriction. Triple split Gaussian type basis sets for
Br and I atoms are obtained from the Extensible Computational Chemistry Environment
Basis Set Database, Pacific Northwest Laboratory.
4.3. Results and Discussion
4.3.1. Structure
Solvent H 2 O molecules are added to Cl 2 , Br 2 and I 2 species by various different
possible ways and full geometry optimization is carried out at BHHLYP/6-311++G(d,p)
level (6-311 basis set is used for I) of theory followed by single point energy calculation
at second order Moller-Plesset perturbation theory (MP2) with the same set of basis
functions. Optimized structures of the most stable configurations for each size hydrated
cluster of Cl 2 , Br 2 and I 2 systems are displayed in Fig. 4.1. Structures corresponding to
IA, IB and IC are the most stable structures of the monohydrate clusters, Cl 2 .H 2 O,
Br 2 .H 2 O and I 2 .H 2 O. For all the structures, O atom from H 2 O molecule is connected to
one of the halogen atoms (r O..X ~2.5 Å). Most favoured structures for the di-hydrated
cluster of Cl 2 , Br 2 and I 2 systems are shown as IIA, IIB and IIC, respectively in Fig. 4.1.
It is clearly observed that only one H 2 O molecule is in the close vicinity of halogen
moiety in the di-hydrated clusters except for Br 2 .2H 2 O cluster, where both the water
molecules are in the close vicinity of Br 2 . It is to be noted that the initial guess structures
for di-hydrated clusters were considered, where one H 2 O molecule attached to one of the
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